1. Technical Field of the Invention
The present invention relates generally to a curable cyanate resin composition having enhanced fracture resistance as a result of the incorporation of reactive thermoplastic oligomers therein. More specifically, the present invention relates to a composition comprising dicyanate ester resins containing at least one thermoplastic polymer modifier which is soluble in the dicyanate ester resin. Said thermoplastic polymer undergoes an in-situ phase separation process during cure to form a microphase-separated multiphase thermoset material.
Accordingly, the present invention embodies low dielectric constant materials with adjustable properties such as glass transition temperature and fracture toughness. The modified cyanate resin has a low dielectric constant and when impregnated into various types of reinforcements, yields insulating materials with either a controlled coefficient of thermal expansion (CTE) or a very low dielectric constant or both.
Furthermore, this invention relates to a curable material useful in conducting heat or electricity comprising a modified cyanate resin material and inorganic or metal particles which exhibits high thermal stability, adjustable glass transition temperature with controlled CTE.
Furthermore, the present invention relates to a curable material useful in the fabrication of prepreg layers for use in the manufacture of electronic packaging structures, adhesives and aerospace structural articles.
Furthermore, this invention relates to a material for use in electronic packaging applications where a thermoset material is required which has low dielectric constant and adjustable properties such as glass transition temperature and fracture toughness, and a process for making the material.
More particularly, this invention relates to a modified cyanate resin material useful in the fabrication of printed circuit boards, semiconductor chip carriers, metal-core boards, reaction injection molded (RIM) cards, multichip modules, and multilayer thin film circuit structures, which may include more than one conductive layer and may incorporate electrical interconnections including through-holes or vias between two or more conductive layers or blind vias between two layers. This invention is well suited for use as a substrate material for surface mounted electronic components.
In addition, this invention relates to an improved material and printed circuit board made therefrom comprising a modified cyanate resin material and reinforcement which exhibits high thermal stability, adjustable glass transition temperature, flame retardancy with either a controlled CTE, or low dielectric constant or both.
2. Prior Art
Polycyanurate thermosets based upon certain cyanate ester resins possess a number of attractive properties such as low dielectric constant, excellent thermal stability, low moisture uptake, high glass transition temperature, and processability characteristics analogous to epoxy resins (melt or solution processable).
Other attractive properties of these polycyanurate thermosets include, methyl ethyl ketone solubility, no volatile formation upon curing, outstanding adhesive properties, photoimageability (if desired) and inherent flame retardancy.
However, use of these polycyanurate thermoset materials in electronic packaging applications (e.g., prepregs, laminates, circuit boards) is limited due to their brittle nature which makes them susceptible to cracking when stressed or during processing such as via formation. This inherent brittleness is due to the high crosslink density of the networks which results in poor fracture toughness. Although there have been significant efforts to enhance the fracture toughness of the aforementioned polycyanurates, further improvement in the mechanical properties of these materials is required in order to make them useful in packaging. (As used herein, "fracture toughness" is a measure of how much energy is needed to propagate a crack in the plastic.)
U.S. Pat. No. 4,157,360 to Prevorsek et al., discloses a composition containing a crosslinked polycyanurate network in which a high molecular weight polyester carbonate, is finely dispersed.
This reference is similar to the present invention in that a thermoplastic is chemically incorporated into a polycyanurate thermoset to enhance the physical properties.
In Prevorsek et al., the chemical structures of both the cyanate ester resins and the thermoplastic modifier are different from those disclosed in the present invention. The present invention discloses highly fluorinated materials that possess high T.sub.g 's, low dielectric constants, inherent flame retardancy, and methyl ethyl ketone solubility.
Prevorsek et al. describe the use of solvent "to cause simultaneous precipitation of the thermoplastic polymer and dicyanate monomer."
Contrary to Prevorsek et al., the present invention describes for the first time, a fluorine-containing thermoplastic polymer (i.e., a modifier) which actually dissolves in the dicyanate monomer resin. So essentially, the dicyanate is a solvent for the modifier polymer. This phenomenon is one key to obtaining microphase separation during thermal curing of the composition of the present invention and hence the resulting improved mechanical properties. This is different from the approach utilized by Prevorsek et al. where what phase separation that does occur, occurs during solvent evaporation, well before the thermal cure. This is an important distinguishing feature that allows for the very small phases present in the present invention.
U.S. Pat. No. 4,902,752 to Shimp discloses toughening polycyanurates with thermoplastics. Shimp discloses other relevant prior art references therein, the contents of which are hereby incorporated by reference herein. More particularly, Shimp discloses curable compositions made from blends of polycyanate esters of polyhydric phenols in admixture with amorphous, aromatic thermoplastic resins which are initially soluble in the polycyanate ester but which phase separate during curing. The reactant materials of Shimp will not function according to the present invention because they are difficult to process and the solvents used inherently raise environmental concerns. In the present invention the chemical structures of both the cyanate ester resin and the thermoplastic are different from those disclosed by Shimp et al.
European Patent Application 0 412 827 A2 to Machenzie et al., discloses a fiber reinforced resin composition containing a polyarylsulfone thermoplastic compound and a cyanate ester resin. This reference is similar to the present invention in that a thermoplastic is chemically incorporated into a polycyanurate thermoset to alter the physical properties. One difference between the present invention and the reference is that the chemical structures of both the cyanate ester resins and the thermoplastic modifier are different. The present invention embodies highly fluorinated materials that possess high T.sub.g 's, low dielectric constants, inherent flame retardancy, and methyl ethyl ketone processability. The resin composition described by Mackenzie et al. is not methyl ethyl ketone soluble, does not possess flame retardant properties and does not possess an inherently low dielectric constant. These attributes are unique to the present invention and are essential for electronics applications where solution processing is utilized.
U.S. Pat. No. 4,745,215 to Cox et al. discloses that dicyanate diphenyl hexafluorinated alkanes can be impregnated into suitable reinforcing fabrics and heat cured at elevated temperatures above their glass transition temperatures, i.e., about 320.degree. C. for 1 hours. The cured products have favorable properties for high temperature and/or electrical insulation uses, such as in laminated circuit boards. These properties include low dielectric constants, high glass transition temperatures and high thermal degradation temperatures. However, it is also recognized that such systems have low fracture resistance and therefore need to be improved for use in electronic packaging.